Test gcc.target/arm/sync-1.c specifies -march=armv7-a and fails
execution for multilibs whose hardware or simulator doesn't support that
architecture.  The test uses __sync_fetch_and_add, which GCC doesn't
support for all arm targets.  Using the -march option allows it to
compile if the multilib options don't conflict, but the test fails at
runtime, often with hangs, if the simulator used for testing doesn't
support atomic builtins.  This patch removes the -march option and
restricts the test to targets that support atomic builtins.

I don't yet have an FSF copyright assignment (it's currently in the
hands of the FSF) but this patch is small enough to not require one.  If
this is OK someone else will need to appy it.